In the leading industrial countries, such as the USA, Great Britain, FRG, France and Japan, ultrasonic technology and methods of non-destructive testing have found extensive application. This can be attributed to the fast development of radio engineering and metallurgy, problems of miniaturization of computing equipment and high-speed apparatuses for quality testing of rolled stock.
In this connection extremely acute is the problem of providing such methods and devices for ultrasonic non-destructive testing as to increase testing efficiency, because employment of current methods and devices keeps the production process back and requires employment of apparatuses and expensive equipment outside the production process at some test zones.
Methods and devices should be developed to increase the reliability of testing, because the existing methods and devices, which require reliable acoustic contact between the radiator and the surface of the object under test, do not ensure reliable detection of flaws; demand more complicated electronic equipment and can discard workpieces with water drops, oil spots and other dirty patches on their surface. Workpieces with peeling scale cannot be tested at all.
Development of new methods and devices is also dictated by the necessity of increasing the sensitivity of testing, and non-destructive testing at high (over 100.degree. C) and low (down to -50.degree. C) temperatures of workpieces.
It is an important problem to provide methods and apparatuses ensuring development of ultrasonic delay lines with a smooth adjustment of the signal delay and an adequate number of tappings.
Methods and devices should be developed to produce dispersionless ultrasonic delay lines which ensure signal transmission without distortions.
At present there are known methods and apparatuses for generation and detection of ultrasonic plate waves, which are employed in ultrasonic engineering and, in particular, in ultrasonic inspection. They include methods and devices for excitation and detection of ultrasonic Lamb waves or longitudinal plate waves based on piezo-effect.
In these known methods and devices plate waves are generated and detected by inducing normal or tangential disturbances on the surface of a workpiece by means of piezoelectric transducers.
There are known several methods for exciting and detecting longitudinal plate waves (Lamb waves):
(1) by normal disturbances, induced by crystal or ceramic transducers uniformly in some area of the workpiece surface, PA1 (2) by a combination of normal disturbances, distributed periodically over the surface of the workpiece and having a period equal to the length of the generated plate wave, the so-called "comb" array method, PA1 (3) by normal disturbances, distributed according to the principle of a sine running wave over the surface of the workpiece -- the "wedge" method.
All these methods are realized providing there is an acoustic contact of piezoelectric transducers with a workpiece. The first of the forementioned methods is practically not used because of its low efficiency and nonresonance nature (all possible types (modes) of plate waves at this frequency are generated in a workpiece). The "comb" method and the "wedge" method are resonance methods. The first permits excitation of a given mode by changing the distance between the teeth of the comb structure. The second permits such excitation by changing the angle of the piezoelectric transducer inclination with respect to the surface of the workpiece. The third of the discussed methods (the "wedge" method) has found application in ultrasonic engineering and in ultrasonic non-destructive testing.
Known devices realizing the "wedge" method comprise a piezoelectric transducer bonded to a prism made of perspex or organic glass and placed in a casing and a device ensuring acoustic contact with the surface of workpieces. When different types of waves are to be excited, a device is introduced for changing the inclination angle of the piezoelectric transducer with respect to the surface of the workpiece. In automatic inspection the device for excitation and reception of plate waves becomes much more complicated. Thus, automatic inspection of sheets in the production process at a speed of 5 m/sec is effected by means of a "wheel" contrivance containing the piezoelectrical transducer placed in a rubber wheel filled with the transformer oil. The device has many complicated parts, and oil is to be fed under the "wheel" in the process of testing to ensure acoustic contact.
In spite of wide application of the discussed methods and devices in ultrasonic flaw inspection, thickness gauging and testing the structure of the workpiece material, these methods and devices possess a number of serious drawbacks. Among them is the necessity of using coupling mediums and, consequently, dependence of the test results on the quality of acoustic contact and a low speed of inspection. Since water is most often used for acoustic contact, requirements to its purity are high, and the temperature of test objects is limited to the above-zero region, in particular it is kept between the freezing point 0.degree. C and the boiling point 100.degree. C, since steam upsets the acoustic contact. The use of adhesive for bonding the piezoelectric transducer to the prism can be the reason of quick failure of the device due to breaking of bonding, if the device operates in conditions of high environmental temperature.
And, finally, the presence of a normal component of the elastic vibrations displacement in the longitudinal plate wave results in reflected signals, which amplitude is comparable to that of signals caused by flaws of the object under test, if minute drops of water and oil stay on the surface of this test object. This leads to discarding fit workpieces and reduces reliability of inspection. Since practically all modes of longitudinal plate waves display dispersion (different components of the frequency spectrum propagate in a workpiece at different velocities and the pulse becomes wider, whereas its amplitude is additionally reduced), it is hard to use them for non-destructive testing. Modes are more effectively excited at the steep portions of dispersion curves of phase and group velocities, when piezoelectrical transducers are used, because the normal components of elastic vibrations displacement are great enough, but can be used only at small bases of sounding workpieces, because the influence of the dispersion sharply weaken and widen the pulses.
The duration of the pulse increases so that the untested edge of the sheet, which becomes the dead zone, can reach 0.2 m and more. The influence of dispersion is less at gently sloping portions of dispersion curves, but excitation of modes is ineffective due to small normal displacement components. At certain horizontal portions of dispersion curves of higher order modes signals cannot be excited at all by means of piezoelectric transducers. It is impossible to excite a zero antisymmetric mode of Lamb waves by the "wedge" method (with an organic glass prism).
All this results in more complicated techniques of selection of work points for flaw inspection and thickness gauging, more complicated and more expensive constructions of apparatuses, their worse technical characteristics and substantial losses in production.
There are known methods of excitation and reception of transverse (displacement) plate waves, which are also called SH-waves, by means of piezoelectric transducers:
1. by displacement disturbances, distributed uniformly over the surface of a workpiece in the band equal to the size of Y-cut crystal or piezoceramic transducers of respective polarization.
2. by a combination of displacement disturbances, distributed periodically over the surface of a workpiece with a space period equal to the length of excited transverse wave, that is the "comb" method.
Since transverse plate waves have no normal displacement components (displacements in transverse plate waves are parallel to surfaces of workpieces), devices for excitation and reception use piezoelectric transducers made of Y-cut crystal or piezoceramic plates of respective polarization. The acoustic contact between the piezoelectric transducer and the workpiece is achieved by bonding the transducer to the workpiece by means of viscous resins or similar substances (ceresine wax, etc.) which ensure transmission of displacing disturbances. This excludes employment of liquids as coupling mediums (water and the like), which does not permit employment of the discussed method for excitation and reception of transverse plate waves in motion, that is during relative movement of workpieces and devices for generation of such waves.
In this connection, at present transverse plate waves are not employed in ultrasonic flaw detection at all, but are used only in ultrasonic delay lines.
There are known a method and a device for generation and detection of Lamb waves (longitudinal plate waves), Rayleigh waves and others (cf. U.S. Pat. No. 3,850,028 Cl. 73/638) based on employment of "contactless" transducers and eliminating the drawbacks caused by the necessity of making an acoustic contact.
This method consists in that to generate and detect ultrasonic Lamb waves two separate transducers are used, each containing a serpentine conductor positioned parallel to the surface of the test object and in the field of a permanent magnet. Alternating current flowing in the conductor induces eddy currents in the test object, in directions transverse to the field of the permanent magnet. The interaction of eddy currents and the field of the permanent magnet result in elastic displacements, which are caused by Lorentz forces (ponderomotive forces) or magnetostriction. Since the serpentine conductor contains a number of parallel portions positioned across the magnetic field and the direction of currents in each parallel portion at any moment is opposite, Lamb waves are excited at the frequency, the product of which by the period (pitch) of the conductor coil is equal to the phase velocity of the elastic wave. In this case Lamb waves are emitted in two opposite directions perpendicular to the conductors.
A device for realization of the discussed method is a carriage mounted on wheels which are driven by a suitable motor mounted on the carriage to make the inspection device travel along a test object.
Two identical transducers mounted on the carriage in proximity with, but spaced from the test object, parallel to each other, one serving as a transmitter and the other as a receiver, are provided with a horseshoe-type permanent magnets, serpentine conductors being positioned between their poles. The carriage also mounts a power supply and a recorder to register the test results. In another embodiment of the device the test object and the serpentine conductor are placed between the poles of the permanent magnet.
But the discussed method and device possess a number of drawbacks related to the nature of Lamb waves and construction peculiarities. These drawbacks, dealt with in detail before, relate to poor reliability of testing because of the influence of minute drops of liquid and other dirt spots of the surface of the workpiece. Dispersion results in the fact that the use of separate transmitting and receiving transducers with an equal pitch (the distance between adjacent coil loops) impairs the test results, since the wave length and the pitch of the receiving transducer are determined disregarding the effect of dispersion on the parameters of the propagating wave. The discussed method and device do not ensure a signal reflected from flaws proportional to the size of these flaws. The method and device under discussion do not permit excitation and detection of transverse plate waves which possess certain advantages as compared to Lamb waves.
Besides, the device provided with separate transmitting and receiving transducers is almost twice more bulky and complicated in servicing, requires replacement of transducers and complete readjustment of equipment for measuring thickness of a test object, which makes its operation much more complex. The transducers, the permanent magnet, the supply source and the recorder placed on one carriage make it heavy which presents quite a problem when testing thin workpieces, deteriorate operating conditions of electronic units and the recorder, reduce the efficiency of testing.
There is also known a device for "contactless" generation and detection of transverse plate waves in testing workpieces (the USSR Author's Certificate No. 411,369 Cl. G01n/29/00, G01B 17/00) which is an electromagnetic acoustic tranducer comprising a magnet and an inductance coil made as a flat spiral and positioned between the test object and the magnet. To excite transverse plate waves the working butt of the magnet of the described device is made as a comb structure (alternating projections and depressions). The sensitivity is increased at the expense of a second comb structure made on the working butt of the magnet, the projections of one structure being in coincidence with the middles of depressions of the other structure). The coil is placed so that its one side is over one comb structure and the second side is over the other structure. The device ensures excitation of transverse plate waves in a workpiece without coupling mediums and at a great speed of movement of the workpiece.
But still, the device possesses some serious drawbacks. Among them are complicated construction of the device, the necessity of a large set of magnet pole shoes with different pitches of the comb structure, complicated adjustment of the device to a new thickness of a workpiece and its new surface shape, technological difficulties related to making pole shoes for excitation transverse plate waves in pipes of different diameters. Besides, resonance properties of the device sharply deteriorate, when testing workpieces made from ferromagnetic materials, which reduces the effectiveness of excitation of the required mode of normal waves.
Apart from the above described devices, there is known a device for generation of plate waves (cf. USSR Author's Certificate No. 375,546 Cl. GO1n 29/04) comprising a casing, wherein an electromagnet is mounted. An inductance coil with evenly spaced sections is placed between the poles of the electromagnet. To increase sensitivity the inductance coil is made as a frame with curved slots, which are parts of a circle. The frame can be turned. The device increases sensitivity of testing due to focussing of ultrasonic vibrations.
The device's drawback consists in that the electromagnet and the inductance coil (frame) are arranged in one casing. This makes the transducer heavy, the access to the frame is difficult, when it needs to be replaced, and the transducer is bulky and inconvenient for testing thin-walled workpieces. The device does not permit optimal excitation of required modes of plate waves and optimal focusing conditions.
There is also known a method of excitation and detection of transverse plate waves in ultrasonic delay lines (cf. "Physical Acoustics" ed. Mason, Vol. 1, Methods and Devices for Ultrasonic Inspection, part A, Moscow, 1966, pp. 513). The ultrasonic delay line comprises a metallic band, which ends are bonded to piezoelectric transducers placed in perpendicular or inclined position with respect to its axis. The side surfaces of the band are insulated by a soundproof material. The piezoelectrical transducers are made of Y-cut crystal or piezoceramic plates of respective polarization. The discussed delay lines, which are called waveguide lines, have extensive application as memories in computers, for aircraft and navigational systems, in radio detection and ranging for pulse compression.
These devices are deficient in that it is impossible to smoothly adjust the delay time and create multi-branch delay lines.